Andreas, you might be misinterpreting Davis. Where did you see the 0.50 in BTZS?

Frequently the ZS variables are 1.20 NDR and 2.10 subject luminance range which is 1.20 / 2.10 = 0.57. While this version of the ZS average gradient is practically identical to Kodak's (and the standard model), the justification for it doesn't reflect what is actually occurring.

Here's a comparison of developmental models from Way Beyond Monochrome, the Zone System, no flare, and my Practical Flare model.